With Internet telephony the terminals of the subscribers establish a connection (Internet Protocol connection) to the device of the partner subscriber and send the voice data as encoded packets over the Internet.
So that the network can set up, monitor and end connections it is necessary to exchange signaling data as well as subscriber or payload data. Basically a distinction is made here between a signaling channel intended to be used for transmission of the above signaling data and a user channel, also referred to as bearer or medium.
There are two signaling models: One is Channel Associated Signaling, CAS, also called inband signaling, in which the signaling data is transmitted over the same network as the payload data and the other is Common Channel Signaling, CCS, also called outband signaling, in which the signaling data is transmitted over a separate network. The advantage of this is that the signaling channel can be optimized in respect of data transmission to provide a higher data transmission rate overall. Signaling information can also be exchanged during an existing session or connection.